U.S. patent number 4,892,475 [Application Number 07/281,459] was granted by the patent office on 1990-01-09 for ignition system and method for post-mixed burner.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to Dennis R. Farrenkopf, William J. Snyder.
United States Patent |
4,892,475 |
Farrenkopf , et al. |
January 9, 1990 |
**Please see images for:
( Certificate of Correction ) ** |
Ignition system and method for post-mixed burner
Abstract
An ignition system and method for a post-mixed burner
characterized by an igniter comprising a tube for igniter oxidant
flow and having an electrode therein, said tube positioned within
the fuel passage but recessed from the discharge end of the fuel
passage and from the combustion zone.
Inventors: |
Farrenkopf; Dennis R. (Bethel,
CT), Snyder; William J. (Ossining, NY) |
Assignee: |
Union Carbide Corporation
(Danbury, CT)
|
Family
ID: |
23077397 |
Appl.
No.: |
07/281,459 |
Filed: |
December 8, 1988 |
Current U.S.
Class: |
431/8; 431/263;
431/266 |
Current CPC
Class: |
F23D
14/22 (20130101); F23Q 3/006 (20130101) |
Current International
Class: |
F23D
14/00 (20060101); F23D 14/22 (20060101); F23Q
3/00 (20060101); F23C 005/00 (); F23Q 007/06 () |
Field of
Search: |
;431/8,174,187,263,264,258,266 ;60/39.827 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Green; Randall L.
Attorney, Agent or Firm: Ktorides; Stanley
Claims
We claim:
1. A method for igniting a combustible mixture comprising:
(A) passing main oxidant into a combustion zone from the discharge
end of a main oxidant passage;
(B) passing fuel into the combustion zone, separately from the main
oxidant, from the discharge end of a fuel passage to form a
combustible mixture in the combustion zone;
(C) passing igniter oxidant having an oxygen concentration greater
than that of air through a tube containing an electrode into the
fuel passage at an angle not exceeding 45 degrees to the direction
in which the fuel is flowing toward the discharge end, at a point
recessed from the discharge end of the fuel passage;
(D) creating a spark from the electrode to cause combustion of fuel
and igniter oxidant within the fuel passage;
(E) passing combusting fuel and igniter oxidant from the fuel
passage into the combustion zone to ignite the combustible mixture;
and
(F) terminating the flow of igniter oxidant while maintaining the
flow of main oxidant into the combustion zone.
2. The method of claim 1 wherein the main oxidant is technically
pure oxygen.
3. The method of claim 1 wherein the main oxidant is
oxygen-enriched air.
4. The method of claim 1 wherein the igniter oxidant is passed
through the tube at a flow rate within the range of from 8 to 50
cfh.
5. The method of claim 1 wherein the igniter oxidant is passed
through the tube into the fuel passage proximate to the inside wall
of the fuel passage.
6. The method of claim 1 wherein the igniter oxidant is passed
through the tube into the fuel passage in substantially the same
direction in which the fuel is flowing toward the discharge
end.
7. The method of claim 1 wherein the igniter oxidant has an oxygen
concentration of at least 30 percent.
8. A post-mixed burner comprising:
(A) a main oxidant passage having a discharge end for supplying
main oxidant into a combustion zone;
(B) a fuel passage having a discharge end for supplying fuel into
the combustion zone separately from the main oxidant; and
(C) an igniter positioned within the fuel passage at an angle not
exceeding 45 degrees to the direction of the fuel flow toward the
discharge end, and recessed from the discharge end of the fuel
passage, said igniter comprising a tube in flow communication with
a source of oxidant having an oxygen concentration greater than
that of air, and an electrode positioned within the tube, and
further comprising means to terminate the flow of igniter oxidant
while maintaining the flow of main oxidant into the combustion
zone.
9. The burner of claim 8 wherein the main oxidant passage is a
central passage and the fuel passage is concentrically oriented
around and along the main oxidant passage.
10. The burner of claim 8 wherein the fuel passage is a central
passage and the main oxidant passage is concentrically oriented
around and along the fuel passage.
11. The burner of claim 8 wherein the fuel passage and the main
oxidant passage are oriented side by side.
12. The burner of claim 8 wherein the fuel passage and main oxidant
passage discharge ends are both in substantially the same
plane.
13. The burner of claim 8 wherein the igniter is positioned within
the fuel passage proximate to the inside wall of the fuel
passage.
14. The burner of claim 8 wherein the igniter is positioned within
the fuel passage in the same direction as the fuel flow toward the
discharge end.
15. The burner of claim 8 wherein the electrode is electrically
insulated along its length except for the end portion.
16. The burner of claim 8 wherein the electrode tip is flush with
the end of the igniter tube.
17. The burner of claim 8 wherein the electrode tip is recessed
from the end of the igniter tube.
18. The burner of claim 9 wherein the igniter is recessed within
the fuel passage by from 4 to 12 inches.
Description
TECHNICAL FIELD
This invention relates generally to the field of post-mixed burners
and, in particular, to ignition systems for post-mixed burners.
BACKGROUND ART
A post mixed burner is a burner wherein fuel and oxidant are
delivered in separate passages to a point outside the burner, such
as a furnace or other combustion zone, where the fuel and oxidant
mix and combust.
A problem in the use of post mixed burners is the operation of a
reliable ignition system. Because, in the operation of a post mixed
burner the combustible mixture is formed not within the burner but
in the combustion zone, the ignition system must be within or close
to the combustion zone, thus exposing the ignition system to the
harsh environment of the combustion zone. This is especially the
case where oxygen is used as the oxidant since oxygen burners
typically do not employ a burner block which may provide some
protection from the radiant heat of the furnace.
In addition to the problem of reduced reliability due to the high
heat resulting from the proximity of the ignition system to the
combustion zone, another problem encountered by post mixed burner
ignition systems is degradation of the ignition system causing
compromised operation.
Generally the ignition system comprises some type of electrical
discharge or spark generating device having electric surfaces which
must be kept clean in order to operate properly. The proximity of
the ignition system to the furnace zone exposes such electric
surfaces to corrosive oxidizing atmospheres which create
difficulties in maintaining the electric surfaces clean and intact.
Moreover, impurities in the fuel may hinder the operation of the
ignition system. Such impurities include moisture, which causes
corrosion of the electric surfaces and can short the electrode to
ground, and particulates, either originally within the fuel or as a
result of incomplete combustion, which tend to foul the electric
surfaces. Such impurities cause spark generation to be reduced or
even totally halted requiring a time consuming and expensive
cleaning or replacement of the ignition system.
Accordingly it is an object of this invention to provide a post
mixed burner having an ignition system which need not be within or
next to the combustion zone and, in addition, can remain free of
operation compromising contaminants better than can conventional
ignition systems.
It is another object of this invention to provide a method for
igniting a post-mixed burner with increased reliability over
conventional post mixed burner ignition systems.
SUMMARY OF THE INVENTION
The above and other objects which will become apparent to those
skilled in the art upon a reading of this disclosure are attained
by the present invention one aspect of which is:
A post-mixed burner comprising:
(A) a main oxidant passage having a discharge end for supplying
main oxidant into a combustion zone;
(B) a fuel passage having a discharge end for supplying fuel into
the combustion zone separately from the main oxidant; and
(C) an igniter positioned within the fuel passage and recessed from
the discharge end of the fuel passage, said igniter comprising a
tube in flow communication with a source of oxidant having an
oxygen concentration greater than that of air, and an electrode
positioned within the tube.
Another aspect of the invention is:
A method for igniting a combustible mixture comprising:
(A) passing main oxidant into a combustion zone from the discharge
end of a main oxidant passage;
(B) passing fuel into the combustion zone, separately from the main
oxidant, from the discharge end of a fuel passage to form a
combustible mixture in the combustion zone;
(C) passing igniter oxidant having an oxygen concentration greater
than that of air through a tube containing an electrode into the
fuel passage at a point recessed from the discharge end of the fuel
passage;
(D) creating a spark from the electrode to cause combustion of fuel
and igniter oxidant within the fuel passage; and
(E) passing combusting fuel and igniter oxidant from the fuel
passage into the combustion zone to ignite the combustible
mixture.
As used herein the term "electrode" means any
electrically-conducting material, such as stainless steel, brass,
or tungsten, which enables the discharge of electrical energy at a
specified location which is usually removed from the electric
potential source.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of one embodiment of the
post-mixed burner of this invention wherein main oxidant is
provided into the combustion zone through a central passage and
fuel is provided into the combustion zone through a passage
concentrically oriented around and along the central passage.
FIG. 2 is a cross-sectional view of another embodiment of the post
mixed burner of this invention wherein fuel is provided into the
combustion zone through a central passage and main oxidant is
provided into the combustion zone through a passage concentrically
oriented around and along the central passage.
FIG. 3 is a cross sectional view of one embodiment of the igniter
useful with the post-mixed burner of this invention.
DETAILED DESCRIPTION
The invention may be practiced with any post-mixed burner
configuration wherein fuel and oxidant are supplied into the
combustion zone through separate passages. The invention will be
described in detail with reference to the Drawings which illustrate
one such configuration wherein the fuel and oxidant are provided
into the combustion zone through concentric passages. Other
configurations include, for example, the provision of fuel and
oxidant into the combustion zone through side by side passages.
Referring now to FIG. 1, post-mixed burner 1 comprises central main
oxidant passage 2 in flow communication with a source of oxidant
(not shown), and concentric fuel passage 3 in flow communication
with a source of fuel (not shown). The main oxidant may be air,
technically pure oxygen having an oxygen concentration of at least
99.5 percent, or oxygen-enriched air having an oxygen concentration
greater than 21 percent. Preferably the fuel is a gaseous fuel
examples of which include natural gas, methane, coke oven gas,
hydrogen, propane, carbon monoxide and blast furnace gas.
The fuel and oxidant are passed separately into combustion zone 4
through the discharge ends of their respective passages and form a
combustible mixture within combustion zone 4. The fuel and oxidant
will flow into the combustion zone to effect a firing rate within
the range of from 1.0 to 25.0 million BTU/HR during high fire
conditions and within the range of from 0.25 to 1.0 million BTU/HR
during low fire conditions. As illustrated in FIG. 1, the fuel and
oxidant passages may have their discharge ends flush with the edge
5 of furnace wall 6 which defines the combustion zone. That is,
both of the discharge ends are in the same plane. Alternatively,
one or both of the discharge ends could be recessed from the plane
formed by edge 5.
It is preferred that the discharge end of the fuel passage not
contain any nozzle or other impediment to the flow of fluid from
the fuel passage into the combustion zone. A nozzle may impede the
ignition flame from the igniter from passing into the combustion
zone. Moreover, the nozzle could become clogged causing an
explosive mixture to form within the fuel passage.
The burner of this invention is characterized by an igniter 7
positioned within the fuel passage so that its discharge end is
recessed from the discharge end of the fuel passage preferably by
at least about 4 inches and up to about 12 inches. The igniter
comprises a tube 8 in flow communication with a source of oxidant
(not shown), and an electrode 9 positioned within tube 8. In the
embodiment illustrated in FIG. 1, igniter 7 passes through the side
wall of fuel passage 3 at an angle to the fuel flow so that igniter
oxidant flowing through tube 8 is passed into the fuel passage at
an angle to the direction in which fuel is flowing in the fuel
passage toward the discharge end. If oriented at an angle, the
igniter is angled up to 45.degree. and preferably within the range
of from 5.degree. to 15.degree. of the fuel passage. The igniter
will be described in greater detail with reference to FIG. 3.
Referring now to FIG. 3, igniter 20 comprises tube 21 which is in
flow communication 22 with a source of oxidant (not shown). The
igniter oxidant must have an oxygen content greater than that of
air. If air were employed as the igniter oxidant, the igniter flame
would be stable only at very low flowrates and furthermore would be
very short requiring that the igniter be very close to or flush
with the fuel passage discharge end. Still further, compressed air
sources may have moisture or oil contaminants which would promote
igniter degradation and malfunction. The greater is the oxygen
concentration of the igniter oxidant, the further the igniter may
be recessed from the fuel passage discharge end and thus the
greater protection which may be afforded the igniter. Preferably
the igniter oxidant has an oxygen concentration exceeding 30
percent. If the main oxidant has an oxygen concentration greater
than that of air, the igniter oxidant source may be the same as the
main oxidant source. Typical oxidant sources include oxygen storage
tanks or, for larger flowrate requirements, air separation
plants.
Generally the igniter oxidant passed through the igniter will be at
a flow rate within the range of from 8 to 50 cubic feet per hour
(cfh). This flowrate is generally within the range of from 0.8 to
5.0 percent of the main oxidant flowrate during low fire operation.
Preferably tube 21 is made of metal such as stainless steel or
inconel.
Within tube 21 is electrode 23 which extends along the length of
tube 21 and whose sparking end 24 may be flush with or, as
illustrated in FIG. 3, recessed from the discharge end 25 of tube
21. If recessed, the recession is generally within the range of
from 3/8 to 1 inch. Electrode 23 is held in place within tube 21 by
any suitable means such as by insulated plug 26 illustrated in FIG.
3. Electrode 23 is connected to a source of electric potential 27
(not shown) which is sufficient to cause a spark to be generated at
sparking end 24. An electric transformer is preferably used as a
source of electric potential. It steps up normal electric
potentials (120 volts) to, for example, 6000 volts. This potential
is then transferred to the electrode end 27 by means of a flexible
ignition wire. Other examples of electric potential sources are
capacitive discharge, piezo electric elements, and static charge
generators.
It is preferred that the spark be generated at sparking end 24. In
FIG. 3 there is illustrated one way of accomplishing this wherein
electrode 23 is coated with polytetrafluoroethylene insulation
along its entire length except for the part near sparking end 24,
and furthermore the portion of electrode 23 near the uninsulated
length is further insulated with ceramic insulation 28. The
electrode may also be uninsulated and the air gap between the
electrode and the igniter tube serves to inhibit sparking at other
than the electrode tip. Sparking at the electrode tip is
accomplished by, for example, bending the sparking end 24 toward
tube 21. In this way the spark will arc from electrode 23 to tube
21 at the shortest distance between them, i.e. at sparking end
24.
In operation, igniter oxidant from source 22 is passed through
passage 29 formed by tube 21, through discharge end 25 and into the
fuel passage at a point upstream, i.e. recessed, from the discharge
end of the fuel passage. This causes the formation of a combustible
mixture proximate the discharge end 25. An electric potential is
applied to electrode 23 causing a spark to form at sparking end 24.
The igniter oxidant flowing through the igniter tube pushes the
spark to the tip of the igniter causing the combustible mixture
proximate discharge end 25 to ignite. The combusting fuel and
igniter oxidant are then passed into the combustion zone by the
action of the flowing fuel in the fuel passage and serve to ignite
the combustible mixture in the combustion zone. Once the post-mixed
burner is ignited, the flow of oxidant to the igniter and the
electric potential supply are terminated and the combustion inside
the fuel passage ceases.
FIG. 2 illustrates another embodiment of the burner of this
invention which will now be briefly described. Referring now to
FIG. 2 post-mixed burner 40 comprises central fuel passage 41 in
flow communication with a source of fuel (not shown), and
concentric main oxidant passage 42 in flow communication with a
source of oxidant (not shown). The fuel and oxidant are passed
separately into combustion zone 43 through the discharge ends of
their respective passages which are flush with edge 44 of furnace
wall 45, and form a combustible mixture within combustion zone 43.
Igniter 46 is positioned within fuel passage 41 so that its
discharge end is recessed from the discharge end of the fuel
passage. In the embodiment illustrated in FIG. 2, igniter 46 passes
through the back wall of fuel passage 41, is positioned proximate
to and axially along the inside wall of fuel passage 41, and
discharges the igniter oxidant into the flowing fuel in the same
direction as that of the flowing fuel toward the discharge end. The
positioning of the igniter proximate to the inside wall of the fuel
passage serves to create additional turbulence at the fuel tube
discharge end thus achieving improved burner ignition. Igniter 46
operates in accord with the description set forth with reference to
FIG. 3 and thus a description of its operation will not be
repeated.
The post-mixed burner and ignition method of this invention serve
to address and to overcome the problems of conventional post mixed
burner ignition systems which were discussed above. First, the
igniter is recessed from the fuel passage discharge end and thus is
positioned well away from the combustion zone and the high
temperatures and corrosive oxidizing conditions attendant thereto.
Despite this well spaced positioning, the ignition system provides
reliable ignition because, although the ignition flame is formed
well away from the combustion zone, it is caused to flow to an into
the combustion zone by the flowing action of the igniter oxidant
and of the fuel.
Second, the flow of igniter oxidant within the igniter and over the
electric surfaces such as around the sparking end, especially in
conjunction with an electrode recessed within the igniter tube,
serves to keep the electric surfaces clear of contaminants such as
moisture, particulates and carbon. In this way moisture does not
form on the electric surfaces thus avoiding corrosion and shorting
out of the electrode, and particulates and carbon do not build up
on the electric surfaces thus avoiding fouling. Instead, these
impurities are swept away from the electric surfaces by the action
of the flowing igniter oxidant, and out into the combustion
zone.
Although the post-mixed burner and ignition method of this
invention have been described in detail with reference to certain
embodiments, those skilled in the art will recognize that there are
other embodiments of the invention within the spirit and scope of
the claims.
* * * * *